Free piston Stirling engines usually drive a mechanical load such as a pump or an electrical alternator. Free piston Stirling coolers are usually driven by an electric motor or the like to transfer heat from one place to another, for example from the inside to the outside of a freezer cabinet. Due to fluctuations in load power demands for engines and heat transfer demands for coolers, the Stirling machine must have a power control to match the engine's output or the cooler's thermal transport to the needs of the system with which the machine is cooperating. For example, a free piston Stirling engine driving a load which decreases or increases its power demand at some time, such as an electrical alternator, must increase or decrease engine power output accordingly.
An associated problem occurs if the load on an engine decreases or cooler thermal transport demand decreases because the amplitude of oscillation of the displacer and piston may increase beyond desirable limits, causing collision of internal engine parts and possible damage. Such overstroke results because the energy input to the engine equals the sum of the energy output and the energy losses of the engine. When a load demand decreases, the excess energy no longer coupled to the load tends to drive the displacer to higher amplitude, usually beyond the maximum desired amplitude and can result in a runaway condition. Therefore, it is additionally desirable to limit the amplitude of oscillation of the displacer and piston in the event of a substantial decrease in load demand.
There is, therefore, a need for a means for controlling the power output and limiting the amplitude of a free piston Stirling engine and controlling the thermal transport of a free piston Stirling cooler.